GB2039680A

GB2039680A - Optical systems

Info

Publication number: GB2039680A
Application number: GB7902723A
Authority: GB
Original assignee: Computer & Technical Expo Syst
Current assignee: Computer & Technical Expo Syst
Priority date: 1979-01-25
Filing date: 1979-01-25
Publication date: 1980-08-13
Also published as: GB2039680B

Abstract

An optical system is provided comprising a projector 10 (e.g. cinematographic), a screen 12, a partially transparent mirror 14 positioned to reflect or transmit light from the projector to the screen and simultaneously transmit or reflect respectively light from the screen for viewing, and means 16 for preventing the light from the projector transmitted or reflected respectively by the mirror 14 from being viewed together with the light from the screen 12. The screen 12 may have the contoured shape substantially of a head, the mirror 14 be a flat sheet of plain glass, the mirror 14 be optically between the screen 12 and means 18 defining a viewing aperture, preferably provided by a window of transparent material, tinted grey and provided by a vertical cylinder within which is the screen 12 facing the mirror 14 and behind the screen there is opacity near or of the cylinder. The preventing means 16 may have the form of an absorbing horn. The housing may comprise lower, middle and upper portions into which it can be disassembled. There may be at least two auxiliary mirrors, 30, 32, 36 e.g. arranged to reflect the optical path parallel to itself and positionable to change the length thereof. <IMAGE>

Description

SPECIFICATION Optical systems This invention relates to optical systems.

In the prior art, it is known to project an image on to a screen but the viewer cannot view the screen exactly along the optical path between the projector and the screen. Because of the angular difference between the direction of projection onto the screen and the direction of viewing of the screen, distortion results. This is particularly noticeable with cinematographic film.

Accordingly, the invention provides an optical system comprising a projector, a screen, a partially transparent mirror positioned to reflect or transmit light from the projector to the screen and simultaneously transmit or reflect respectively light from the screen for viewing, and means for preventing the light from the projector transmitted or reflected respectively by the mirror from being viewed together with the light from the screen.

According to another aspect of the invention, there is provided an optical system with means defining positions for the projector and screen if they are not actually present.

Thus, a viewer can see the screen from substantally the same direction as that in which the projector projects light on to the screen. This reduces distortion. In the case that the screen is contoured, especially if it has a shape substantially of a head, e.g. a human head, this also ensures that shadows or low illumination parts of the screen resulting from the projection are not noticeable to the viewer, which provides a greater degree of reality if, for example, the projection is of a cinema tographicfilm in which the position of the lips and the facial expression change. A very realistic result has been found when the mirror is a flat sheet of plain glass. To obtain more reality, the system includes means defining a viewing aperture to reduce the angular dispersion of viewers.For realistic results, the mirror is placed optically between the screen position and such means. For further increasing reality, the aperture is provided by a window of transparent material, which tends to make small imperfections or discrepancies at the screen less apparent. If the window is tinted, e.g. being made of grey-tinted acrylic sheet, this increases the apparent contrast ratio by approximately doubling the internal contrast.

A particularly pleasing appearance, which also heightens the reality, is obtained when the aperture is provided by a vertical cylinder, at least partly made of transparent material. Reality is improved if the screen position is within the cylinder and facing the mirror and there is opacity near or of the cylinder behind the screen positiion, e.g. there is a masking material adjacent the cylinder or the cylinder itself has imprinted on it a screen pattern, e.g. of close dots, e.g. totalling more than 50% opacity.

An efficient construction of the preventing means is to provide the latter in the form of an absorbing horn. Alternatively, the relevant light can be diffused and recirculated as diffuse background illumination to the screen. Again, it could be let out vertically from the system so as not to be viewed together with the light from the screen.

As a matter of constructional convenience, a housing for the system may comprise three portions, a lower portion containing the projector position, a middle portion defining a viewing aperture, and an upper portion comprising the preventing means. For further convenience, especially in transportation, the housing can be disassembled into the three portions.

In order to provide a compact system, it is convenient to introduce in the optical path between the mirror and the projector at least two auxiliary mirrors. If these are arranged to reflect the optical path parallel to itself and are provided with positioning means, they can be used to change the length of the optical path without changing the screen and projector positions. If a third auxiliary mirror is used, the projector can be located at the front of the housing (below the viewing aperture) so that it is easily accessible without operating space being required behind the housing.

Reference will now be made by way of example to the accompanying drawings, in which: Figure 1 is a vertical section through an optical system embodying the present invention; and Figure2 is a schematic diagram of an alternative embodiment.

Referring to the drawings, an optical system comprises a cinematographic projector 10, a screen 12, a partially transparent mirror 14 positioned to reflect light from the projector to the screen and simultaneously transmit light from the screen for viewing, and means 16 for preventing the light from the projector transmitted by the mirror from being viewed together with the light from the screen. The screen 12 is contoured and has the shape substantially of a head. The mirror 14 is a flat sheet of plain glass. The mirror 14 is optically between the screen 12 and a cylinder 18 defining a viewing aperture on the left as seen in Figure 1. The aperture is provided by a window of transparent material which is acrylic and is tinted grey to increase the apparent contrast ratio. The cylinder is vertical and is made of the transparent material.The screen 12 is within the cylinder 18 and facing the mirror 14 and an opaque sheet 20 is located near the cylinder behind the screen position, the front edges of the sheet slanting downwards and approximately following the intersection line of the glass sheet 14 with the cylinder 18. The preventing means 16 has the form of an absorbing horn and it will be apparent how the path 22 of light transmitted through the mirror 14 is multiply reflected in the horn and eventually fully absorbed by material 24 lining the horn, which is wedge-shaped.

A housing for the system comprises three portions, a lower portion 26 (itself comprising upper and lower cylindrical portions) a middle portion defining a viewing aperture and in fact being the aforementioned cylinder 18, and an upper portion 28 comprising the preventing means 16. The various portions fit together and the housing can be disassembled into them.

In the optical path between the mirror 14 and the projector 10, there are at least two front-silvered auxiliary mirrors 30 and 32. These are arranged to reflect the optical path 34 parallel to itself along line 36 and are provided with positioning means 38, 40 (in the form of screws co-operating with horizontal guides 42, 44) to change the length of the optical path without changing the screen and projector positions. A third front-silvered auxiliary mirror 46 can be provided for the reasons indicated above.

Referring to Figure 2, it is alternatively possible for the projector 10 to transmit by projection through the mirror 14to the screen 12 and for light from screen 12 to be reflected through a viewing window 18. In this case, preventing means 16 would be positioned opposite the window 18 so as to absorb light from projector 10 reflected at mirror 14.

The specification also contemplates any workable combination of any of the features mentioned above.

The specification contemplates as inventions any inventive feature or inventive combination of features referred to above.

Claims

1. An optical system for use in projection of images, comprising means defining a first position to accommodate a projector, means defining a second position to accommodate a screen, a partially transparent mirror positioned optically between the first and second positions to reflect or transmit light from the first to the second position and simultaneously transmit or reflect respectively light from the second position for viewing, and means for preventing light from the first position transmitted or reflected respectively by the mirror from being viewed together with the light from the second position.

2. An optical system comprising a projector, a screen, a partially transparent mirror positioned to reflect or transmit light from the projector to the screen and simultaneously transmit or reflect respectively light from the screen for viewing, and means for preventing the light from the projector transmitted or reflected respectively by the mirror from being viewed together with the light from the screen.

3. A system as claimed in claim 1 or 2, in which the screen is present and contoured.

4. A system as claimed in claim 3 in which the screen has the contoured shape substantially of a head.

5. A system as claimed in any preceding claim, in which the mirror is a flat sheet of plain glass.

6. A system as claimed in any preceding claim, in which the mirror is optically between the screen position and means defining a viewing aperture.

7. A system as claimed in claim 6, in which the aperture is provided by a window of transparent material.

8. A system as claimed in claim 7, in which the window is tinted to increase the apparent contrast ratio.

9. A system as claimed in claim 6,7 or 8, in which the aperture is provided by a vertical cylinder at least partly made of transparent material.

10. A system as claimed in claim 9 in which the screen position is within the cylinder and facing the mirror and there is opacity near or of the cylinder behind the screen position.

11. A system as claimed in any preceding claim, in which the preventing means has the form of an absorbing horn.

12. A system as claimed in any preceding claim in which a housing for the system comprises three portions, a lower portion containing the projector position, a middle portion defining a viewing aperture, and an upper portion comprising the preventing means.

13. A system as claimed in claim 12, in which the housing can be disassembled into the three portions.

14. A system as claimed in any preceding claim in which, in the optical path between the mirror and the projector position, there are at least two auxiliary mirrors.

15. A system as claimed in claim 14, in which two said auxiliary mirrors are arranged to reflect the optical path parallel to itself and are provided with positioning means to change the length of the optical path without changing the screen and projector positions.

16. An optical system substantially according to any embodiment hereinbefore described or referred to.